1. Field of the Invention
This invention relates to a method for the manufacture of a silver catalyst to be used in the production of ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen.
2. Description of Prior Arts
It is universally known that virtually all catalysts to be used for the production, on a commercial scale, of ethylene oxide by the catalytic vapor phase oxidation of ethylene with molecular oxygen are silver-deposited catalyst having silver deposited on porous inorganic carriers. It is also a well-known fact that practically all these silver-deposited catalysts have promoters or so-called reaction accelerators deposited other than silver on the carriers for the purpose of acquiring high activity, high selectivity, and high durability as the performance demanded from the commercial point of view.
Although promoters or reaction accelerators sound very simple, they are widely varied. A review of patents which have issued to date in this field reveals that substantially all metallic elements appearing in the Periodic Table of Elements have been involved in some or others of such patents. It is only a small proportion of the numerous promoters or reaction accelerators so far developed that have found actual commercial adoption. Among others, alkali metals which have long been known well as reaction accelerators have recently come to attract increasing attention as evidenced by the great number of recent patent applications which, cover inventions involving the reaction accelerators. Examples are U.S. Pat. No. 3,962,136, U.S. Pat. No. 4,033,903 U.S. Pat. No. 4,066,575, U.S. Pat. No. 4,212,772 and U.S. Pat. No. 4,168,247. The catalysts offered by such inventions have more or less reached the level demanded from the commercial point of view and some of them demonstrate notable performance. Nevertheless, they still have room to be desired or improved. For example, the addition of alkali metals has entailed loss of activity in some inventions and the improvement of selectivity by the addition of alkali metals has fallen short of hitting the target in some other inventions. Particularly, the rapid loss of performance peculiar to alkali metal-deposited silver catalysts and the gradual loss of performance during storage are important problems which await solution.
To be more specific, use of alkali metals as reaction accelerators in silver catalysts for the production of ethylene oxide has a long history as described above. Use of sulfates of such alkali metals as potassium, rubidium and cesium already appeared in the specification of U.S. Pat. No. 2,671,764. Since then, alkali metal-deposited silver catalysts have often been disclosed in patent specifications. The catalysts as disclosed in such patent specifications represented technical achievements surpassing the technical levels prevailing at the respective times of relevant patent applications. Particularly, the catalyst disclosed by U.S. Pat. No. 3,962,136 deserves special attention in respect that, unlike the alkali metal-deposited silver catalysts developed therefore, it has fixed the amount of alkali metal to be added in a specific range and consequently succeeded in immensely improving the selectivity.
Many of the silver catalysts disclosed in the later patent specifications have had the amounts of alkali metal deposition and other various conditions limited with a view to providing improvements in catalyst performance. Catalysts possessed of fairly improved performance have come to appear.
Despite such notable improvements, these catalysts still have room for more selectivity and they are not quite free from the fault of the loss of capacity during use. Particularly the alkali metal-deposited silver catalysts, despite the notable initial improvement in selectivity due to the addition of alkalis, continue to suffer from the fault that the degradation of the performance occurs rapidly. For the catalysts which are expected to offer effective service for a number of years, such rapid loss of performance has a grave economic impact and remains to be an important problem awaiting a solution. When the alkali metal-deposited silver catalysts which have been activated are left standing unused for a prolonged period, say for half a year or one year, for the purpose of storage, for example, they have their activity and selectivity degraded during the standing. This gradual loss of performance can never be overlooked from the practical point of view.
For the explanation of these disadvantages, various factors are conceivable such as migration of alkali metal into silver or carrier, sintering of silver particles, change in chemical state of alkali metal, and poisoning of the catalyst during the use of the catalyst, and poisoning of the catalyst during the preservation. Yet, no definite proof is available for ascertaining the mechanism responsible for the loss of performance.
An object of this invention, therefore, is to provide a method for the manufacture of an improved silver catalyst for the production of ethylene oxide.
Another object of this invention is to provide a method for the manufacture of a silver catalyst of enhanced activity, selectivity, and durability for the production of ethylene oxide.